To answer this question (from a UK perspective ) would require far more time than I've got. But - basically:
There is not just ATC in the Airways and Controlled airspace that the Commercial aircraft use. Some busy light GA airfields have ATC. In addition to this there is the military and a few civil airfields who provided what is known as a "Lower Airspace Radar Service" to aircraft ouside of regulated airspace who want an ATC service to try and keep them out of the way of other aircraft. Useful also as it means that you can inform any aircraft you may have inbound or outbound of any possible conflictions and do something about it.
So - quite a lot of light aircraft use it in one form or another. It all depends where they are.